Heat pipe with sintered powder wick

ABSTRACT

A heat pipe ( 10 ) includes a casing ( 12 ) and a sintered powder wick ( 14 ) arranged at an inner surface of the casing. The sintered powder wick is in the form of a multi-layer structure of at least three layers and each layer has an average powder size different from that of the other layers, wherein the layer ( 143 ) with large-sized powders is capable of reducing the flow resistance to the condensed liquid to flow back while the layer ( 141 ) with small-sized powders is still capable of providing a relatively large capillary force for the wick.

TECHNICAL FIELD

The present invention relates generally to heat pipes as heattransfer/dissipating device, and more particularly to a heat pipe with asintered powder wick.

BACKGROUND

Heat pipes have excellent heat transfer performance due to their lowthermal resistance, and therefore are an effective means for heattransfer or dissipation from heat sources. Currently, heat pipes arewidely used for removing heat from heat-generating components such ascentral processing units (CPUs) of computers. A heat pipe is usually avacuum casing containing therein a working fluid, which is employed tocarry, under phase transitions between liquid state and vapor state,thermal energy from one section of the heat pipe (typically referring toas “evaporating section”) to another section thereof (typicallyreferring to as “condensing section”). Preferably, a wick structure isprovided inside the heat pipe, lining the inner walls of the casing, forwicking the working fluid back to the evaporating section after it iscondensed at the condensing section. Specifically, as the evaporatingsection of the heat pipe is maintained in thermal contact with aheat-generating component, the working fluid contained at theevaporating section absorbs heat generated by the heat-generatingcomponent and then turns into vapor. Due to the difference of vaporpressure between the two sections of the heat pipe, the generated vapormoves towards and carries the heat simultaneously to, the condensingsection where the vapor is condensed into liquid after releasing theheat into ambient environment by, for example, fins thermally contactingthe condensing section. Due to the difference of capillary pressuredeveloped by the wick structure between the two sections, the condensedliquid is then wicked back by the wick structure to the evaporatingsection where it is again available for evaporation.

The wick structure currently available for heat pipes includes finegrooves integrally formed at the inner walls of the casing, screen meshor bundles of fiber inserted into the casing and held against the innerwalls thereof, or sintered powder combined to the inner walls bysintering process. Among these wicks, the sintered powder wick ispreferred to the other wicks with respect to heat transfer ability andability against gravity of the earth.

The primary function of a wick is to draw condensed liquid back to theevaporating section of a heat pipe under the capillary pressuredeveloped by the wick. Therefore, the capillary pressure is an importantparameter affecting the performance of the wick. Since it is wellrecognized that the capillary pressure of a wick increases due to adecrease in pore size of the wick, the sintered powder wick generallyhas a capillary pressure larger than that of the other wicks due to itsvery dense structure of small particles. In order to obtain a relativelylarge capillary pressure for a sintered powder wick, small-sized powderis often used so as to reduce the pore size formed between the particlesof the powder. However, it is not always the best way to choose asintered powder wick based on the size of powder, because the flowresistance to the condensed liquid also increases due to a decrease inpore size of the wick. The increased flow resistance reduces the speedof the condensed liquid in returning back to the evaporating section andtherefore limits the heat transfer performance of the heat pipe. As aresult, a heat pipe with a wick that has too large or too small a poresize often suffers dry-out problem at the evaporating section as thecondensed liquid cannot be timely sent back to the evaporating sectionof the heat pipe.

Therefore, there is a need for a heat pipe with a sintered powder wickwhich can provide simultaneously a relatively large capillary force anda relatively low flow resistance so as to effectively and timely bringthe condensed liquid back from its condensing section to its evaporatingsection and thereby to avoid the undesirable dry-out problem at theevaporating section.

SUMMARY

A heat pipe in accordance with a preferred embodiment of the presentinvention includes a casing and a sintered powder wick arranged at aninner surface of the casing. The sintered powder wick is in the form ofa multi-layer structure of at least three layers and each layer has anaverage powder size different from that of each of the other layers.

The present invention in another aspect, relates to a method formanufacturing a sintered heat pipe. The preferred method includes stepsof: (1) providing a hollow casing and at least three groups of powderwith each group having an average powder size different from that ofeach of the other groups; (2) filling said at least three groups ofpowder sequentially into said casing at a location adjacent to an innersurface of the casing with the later filled group of powder beingstacked on the earlier filled group of powder; and (3) conductingsintering process to the casing and the filled powder, whereby asintered powder wick with a multi-layer structure is formed inside thecasing.

Other advantages and novel features of the present invention will becomemore apparent from the following detailed description of preferredembodiment when taken in conjunction with the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of a heat pipe inaccordance with a first embodiment of the present invention;

FIG. 2 is a longitudinal cross-sectional view of a heat pipe inaccordance with a second embodiment of the present invention;

FIG. 3 is a longitudinal cross-sectional view of a heat pipe inaccordance with a third embodiment of the present invention;

FIGS. 4-5 are longitudinal cross-sectional views showing the steps of apreferred method in manufacturing the heat pipe of FIG. 1; and

FIGS. 6-8 are longitudinal cross-sectional views showing the steps of apreferred method in manufacturing the heat pipe of FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a heat pipe 10 in accordance with a first embodimentof the present invention. The heat pipe 10 includes a casing 12 and acapillary wick 14 arranged at an inner surface of the casing 12. Thecasing 12 includes an evaporating section 121 and a condensing section123 at respective opposite ends thereof, and a central section 122located between the evaporating section 121 and the condensing section123. The casing 12 is typically made of high thermally conductivematerials such as copper or copper alloys. The wick 14 is saturated witha working fluid (not shown), which acts as a heat carrier for carrythermal energy from the evaporating section 121 toward the condensingsection 123 when undergoing a phase transition from liquid state tovaporous state. In more detail, heat that needs to be dissipated istransferred firstly to the evaporating section 121 of the heat pipe 10to cause the working fluid saturated in the wick 14 to evaporate. Then,the heat is carried by the working fluid in the form of vapor to thecondensing section 123 where the heat is released to ambient environmentand the vapor is condensed into liquid. The condensed liquid then isbrought back, via the wick 14, to the evaporating section 121 where itis again available for evaporation.

The capillary wick 14 is a sintered powder wick which is formed bysintering small-sized powder, such as metal powder including copper andaluminum, or ceramic powder under high temperature. Along a longitudinaldirection of the casing 12, the wick 14 has a multi-layer structure,which includes in sequence a first layer 141, a second layer 142 and athird layer 143. In this embodiment, the first, second and third layers141, 142, 143 correspond to the evaporating, central and condensingsections 121, 122, 123 of the casing 12, respectively. Each layer of thewick 14 has an average powder size different from that of each of theother layers. The first layer 141 has the smallest average powder size,whereas the third layer 143 has the largest average powder size. Thatis, the three layers 141, 142, 143 are stacked together in such a mannerthat the average powder sizes thereof gradually increase along thelongitudinal direction from the evaporating section 121 toward thecondensing section 123.

Since the particle size of the powder also determines the pore sizeformed between the particles of the powder, the average pore sizes ofthese layers 141, 142, 143 also increase along the longitudinaldirection from the evaporating section 121 toward the condensing section123. According to the general rule that the capillary pressure of a wickand its flow resistance to the condensed liquid increase due to adecrease in pore size of the wick, the multi-layer construction of thewick 14 is thus capable of providing a capillary pressure graduallyincreasing from the condensing section 123 toward the evaporatingsection 121, and a flow resistance gradually decreasing from theevaporating section 121 toward the condensing section 123. Specifically,the third layer 143 and the second layer 142 have a large average poresize and therefore provide a relatively low resistance to the condensedliquid to flow back, thereby effectively reducing the barriers thecondensed liquid encounters in returning back from the condensingsection 123 toward the evaporating section 121. The first layer 141,however, is constructed to have a small average pore size in order tostill maintain a relatively high capillary force for the wick 14.Therefore, the wick 14 is capable of providing simultaneously arelatively low flow resistance to the condensed liquid when it flows atthe condensing and central sections 123, 122 and a relatively highcapillary pressure for drawing the condensed liquid back from thecondensing and central sections 123, 122 toward the evaporating section121. As a result, the condensed liquid is timely brought back to theevaporating section 121 in an accelerated manner, thereby effectivelyavoiding dry-out problem happening at the evaporating section 121.

FIG. 2 illustrates a heat pipe 20 according to a second embodiment ofthe present invention. The heat pipe 20 includes a casing 22 and acapillary wick 24 arranged at an inner surface of the casing 22. Thewick 24 is in the form of a multi-layer structure which includes anouter layer 241, an intermediate layer 242 and an inner layer 243. Theselayers 241, 242, 243 are stacked together along a radial direction ofthe casing 22 with the outer layer 241 being connected to the innersurface of the casing 22. Each layer of the wick 24 has an averagepowder size different from that of the other layers, and these layers241, 242, 243 are stacked in such a manner that the average powder sizesthereof gradually increase along the radial direction from the innersurface of the casing 22 towards a central axis X-X of the casing 22.Since powder size defines pore size between particles of the powder, theaverage pore sizes of these layers 241, 242, 243 also gradually increasefrom the inner surface of the casing 22 towards the central axis X-X ofthe casing 22. According to the above-mentioned general rule that thecapillary pressure of a wick and its flow resistance to the condensedliquid increase due to a decrease in pore size of the wick, the innerlayer 243 and the intermediate layer 242 have a large average pore sizeand therefore are capable of providing a relatively low resistance tothe condensed liquid to flow back. The outer layer 241, however, has asmall average pore size and therefore is still capable of maintaining arelatively high capillary force for the wick 24. Thus, the multi-layerconstruction of the wick 24 is capable of providing between these layers241, 242, 243 along the radial direction of the casing 22 a gradient ofcapillary pressure gradually increasing from the central axis X-X of thecasing 22 toward the inner surface of the casing 22, and a gradient offlow resistance gradually decreasing from the inner surface of thecasing 22 toward a central axis X-X of the casing 22. Furthermore, thesmall-sized outer layer 241 of the wick 24 is also capable ofmaintaining an increased contact surface area with the inner surface ofthe casing 22, as well as a large contact surface with the working fluidsaturated in the wick 24, to thereby facilitate heat transfer betweenthe heat pipe 20 and a heat source outside the heat pipe 20 that needsto be cooled.

FIG. 3 illustrates a heat pipe 30 according to a third embodiment of thepresent invention. Similar to the second embodiment, the heat pipe 30also has a multi-layer powder-based capillary wick 34 arranged at aninner surface of the heat pipe 30. The wick 34 includes an outer layer341, an intermediate layer 342 and an inner layer 243 which are stackedalong a radial direction of the heat pipe 30 with the outer layer 341being connected to the inner surface of the heat pipe 30. These layers341, 342, 343 have different powder sizes to each other. In contrary tothe second embodiment, these layers 341, 342, 343 are arranged in anorder that the powder sizes thereof gradually decrease from the innersurface of the heat pipe 30 towards a central axis Y-Y of the heat pipe30. Therefore, the large-sized outer layer 341 has a relatively largepore size and accordingly develops a relatively low resistance to thecondensed liquid to return back. However, this construction of the wick34 is suitable for heat pipes with relatively short lengths, incomparison with the wick 24.

The heat pipe 10 as disclosed in the first embodiment can be made byusing the method as illustrated in FIGS. 4-5. In order to form themulti-layer capillary wick 14, three groups of powder are prepared inadvance with each group having an average powder size different fromthat of the other groups. Firstly, a mandrel 40 is inserted into thecasing 12 with a space 50 formed between the casing 12 and the mandrel40. The three groups of powder are sequentially filled into the space 50with the later filled group of powder being stacked on the earlierfilled group of powder along the longitudinal direction of the casing12. The three groups of powder are filled into the casing 12 in such anorder that the powder size of the later filled group of powder is largerthan that of the earlier filled group of powder so as to prevent theparticles of the later filled group of powder from falling into thespaces formed between the particles of the earlier filled group ofpowder. After all of these groups of powder are filled into the casing12, the casing 12 with the three groups of powder is subject to heatwith a high temperature, fox example, about 950 degrees Celsius when thepowder is copper powder, to thereby sinter the three groups of powdertogether whereby the heat pipe 10 with the multi-layer sintered powderwick 14 arranged along the inner surface of the casing 12 is obtained.

The heat pipe 20 as disclosed in the second embodiment can be made byusing the method as illustrated in FIGS. 6-8. Three groups of powderwith different powder sizes from each other are filled sequentially intothe casing 22 at a location adjacent to the inner surface of the casing22 with the later filled group of powder being stacked on the earlierfilled group of powder along the radial direction of the casing 22. Thethree groups of powder are filled into the casing 22 in the order thatthe powder size of the later filled group of powder is larger than thatof the earlier filled group of powder. When filling each group ofpowder, a mandrel is used to control the thickness thereof. For example,as illustrated in FIG. 6, a first mandrel 40 a is inserted into thecasing 22 to control the thickness of the first group of powder, whichis to be formed as the outer layer 241 of the wick 24. Likewise, secondand third mandrels 40 b, 40 c with smaller diameters than that of thefirst mandrel 40 a are respectively and successively used to control thethicknesses of the second and third groups of powder, which are to beconstructed as the intermediate and inner layers 242, 243 of the wick22, respectively. In order to keep the powder in place and prevent,after the corresponding mandrel 40 a, 40 b or 40 c is drawn out, thepowder from dropping into the hollow space that is originally occupiedby the corresponding mandrel 40 a, 40 b or 40 c, each group of powder ispre-sintered at a suitable temperature, for example, about 630 degreesCelsius in the context of copper powder, before the correspondingmandrel 40 a, 40 b or 40 c used to control the thickness thereof isdrawn out of the casing 22. Finally, all of these groups of powderfilled into the casing 22 together with the casing 22 are sintered undera high temperature, for example, about 950 degrees Celsius to therebyobtain the heat pipe 20 with the multi-layer sintered powder wick 24arranged in the casing 22. It is apparent that, if the filling order ofthe three groups of powder into the casing 22 is reversed, this methodis also suitable for manufacturing the heat pipe 30 of the thirdembodiment. In this situation, in order to prevent the later filledsmall-sized powder from falling into the spaces defined betweenparticles of the former filled large-sized powder, partitioning meanssuch as a layer of polymeric bonding agent can be applied between everytwo adjacent groups of powder. However, the bonding agent can bedecomposed by subsequently applying heat thereto.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size, and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

1. A heat pipe comprising: a casing; and a sintered powder wick arrangedat an inner surface of the casing; wherein the sintered powder wick isin the form of a multi-layer structure of at least three layers and eachlayer has an average powder size different from that of each the otherlayers.
 2. The heat pipe of claim 1, wherein said at least three layersare stacked in such a manner that the average powder sizes thereofincrease along a predetermined direction.
 3. The heat pipe of claim 2,wherein said direction is a radial direction of the casing.
 4. The heatpipe of claim 2, wherein said direction is a longitudinal direction ofthe casing.
 5. The heat pipe of claim 1, wherein the sintered powderwick is one of a sintered metal powder wick and a sintered ceramicpowder wick.
 6. A method for manufacturing a heat pipe comprising stepsof: providing a hollow casing and at least three groups of powder witheach group having an average powder size different from that of each ofthe other groups; filling said at least three groups of powdersequentially into said casing at a location adjacent to an inner surfaceof the casing with the later filled group of powder being stacked on theearlier filled group of powder; and conducting sintering process to thecasing and the filled powder, whereby a sintered powder wick with amulti-layer structure is formed inside the casing.
 7. The method ofclaim 6, wherein said at least three groups of powder are filled intosaid casing in such an order that the average powder size of the laterfilled group of powder is larger than that of the earlier filled groupof powder.
 8. The method of claim 6, wherein a mandrel is used to beinserted into said casing with a space formed between the casing and themandrel, and said at least three groups of powder are filled into saidspace to be stacked along a longitudinal direction of the casing.
 9. Themethod of claim 6, wherein said at least three groups of powder arestacked along a radial direction of the casing, and when filling eachgroup of powder, a mandrel is used to control a thickness thereof. 10.The method of claim 9 further comprising a step of pre-sintering eachgroup of powder before a corresponding mandrel used to control athickness thereof is drawn out of the casing.
 11. A heat pipecomprising: a metal casing having an outer surface and an inner surface;a wick formed on the inner surface of the metal casing; and workingfluid received in the metal casing and capable of becoming into vaporupon receiving heat at a first place of the heat pipe and liquid uponreleasing the heat at a second place of the heat pipe, the liquid beingdrawn by the wick from the second place of the heat pipe to the firstplace of the heat pipe; wherein the wick forms capillary pressure indrawing the liquid, the capillary pressure being in gradient along oneof radial and longitudinal directions of the heat pipe.
 12. The heatpipe of claim 11, wherein the wick is formed by sintering powders havingdifferent powder sizes.
 13. The heat pipe of claim 12, wherein thepowders have three different sizes, with the powders having the largestsize being located near the second place of the heat pipe and thepowders having the smallest size being located near the first place ofthe heat pipe.
 14. The heat pipe of claim 12, wherein the powders havethree different sizes, with the powders having the largest size beinglocated close to the casing and the powders having the smallest sizebeing located close to a center of the heat pipe.
 15. The heat pipe ofclaim 12, wherein the powders have three different sizes, with thepowders having the largest size being located close to a center of theheat pipe and the powders having the smallest size being located closeto the casing.